A more or less effective luminescence conversion has already been used for some time in various fields, for example in radiation detector technology. In general, functional units that are used for luminescence conversion are based on absorption/emission processes. Utilized is the fact that there is a shift of luminescence to longer wavelengths compared to absorption in most cases, for energetic reasons. This phenomenon can be used, for example, for spectral matching of detector sensitivity to a radiation source. Furthermore, the property of luminescence radiation no longer to be bound to the direction of the incident radiation is of interest, since concentration of radiation in a medium can be realized by total reflection at the interfaces.
A recent example is the production of “white” light by way of partial conversion of the radiation from a blue luminescent diode. The LUCOLED (P. Schlotter, R. Schmidt, J. Schneider, Appl. Phys. A 64, 417 (1997)) utilizes this principle. A portion of the high-energy blue luminescent radiation is absorbed by a suitable layer in the beam direction and is emitted again shifted toward lower energies, so that a white color impression is produced by additive mixing.
DE 196 25 622 A1 describes such a light-radiating semiconductor component with a semiconductor body emitting radiation and with a luminescence conversion element. The semiconductor body has a sequence of semiconductor layers that emits electromagnetic radiation with a wavelength λ of ≦520 nm, and the semiconductor conversion element converts radiation of a first spectral subregion of the radiation emitted by the semiconductor body from radiation originating from a first wavelength region into radiation of a second wavelength region, so that the semiconductor component emits radiation from a second spectral subregion of the first wavelength region and radiation of the second wavelength region. Thus, for example, radiation emitted by the semiconductor body is absorbed with spectral selectivity by the luminescence conversion element and is emitted in the longer-wavelength region (in the second wavelength region). In this method, organic dye molecules are imbedded in an organic matrix.
DE 196 38 667 A1 also discloses a semiconductor component with a semiconductor body emitting radiation and a luminescence conversion element that emits mixed-color light, with the luminescence conversion element having a luminous inorganic substance, in particular a phosphor.
Besides spectral suitability with regard to the corresponding application, such a layer has two principal requirements: The photoluminescence quantum yield must be high, usually clearly greater than 50%, and its stability must permit long service lives, usually more than 10,000 hours.
The basic concept for realizing such a layer with organic dyes consists of separating and immobilizing molecules in a matrix so that they behave like monomers with optical properties similar to a liquid solution, particularly with high quantum yield. Polymers and sol-gel layers are known as matrices.
Mixed layers that were produced from the organic dyes 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) and SiO2 by co-vaporization onto quartz substrates under high vacuum are described in H. Fröb, K. Kurpiers, K. Leo, CLEO'98, San Francisco/CA, May 1998, 210; 1998 OSA Technical Digest Series Vol. 6, published by Optical Society of America (“The Fröb publication”). The concentration range studied was 0.65–100 vol. %. It was observed that the absorption and emission spectra for decreasing concentrations gradually approach those in a liquid solution, and for the lowest concentration a photoluminescence quantum yield of about 50% is achieved at room temperature (FIG. 6, corresponding to FIG. 2 of the Fröb publication).
A device used for this purpose is described by M. A. Herman, H. Sitter, Molecular Beam Epitaxy, Ch. 2 (Sources of Atomic and Molecular Beams), Springer 1989, pp. 29–59. A dye vaporizer and a metal oxide vaporizer are provided in a vacuum chamber, whose vapor beam is aimed at a substrate, with the dye vaporizer being cup-shaped and consisting, viewed from the inside toward the outside, of a quartz cuvette, a graphite block, a heater, a shield, and a jacket, with a thermocouple being placed between the quartz cuvette and the graphite block in the bottom center of the cup.
FIG. 7 shows the normalized absorption and emission of 30-nm thick layers for a pure and a diluted dye layer. It is important that the spectra of the diluted layers can be fitted to those of monomers with their typical vibrational progression. It is found that the line width remains constant for all low concentrations; its enlargement compared to that observed in liquid solution is not surprising, considering the inhomogeneous conditions of the surroundings of the molecule.
The authors hold a weakening Förster transfer because of the increasing mean molecular separation responsible for the increase of quantum yields toward lower concentrations, and they expect a maximum at about 0.1 vol. %, but of course without experimental confirmation of this. No predictions are made about the lifetime, with regard to which all organic conversion layers so far have foundered.